1. Field of the Invention
The invention concerns an automatic tensioning device for keeping an idler pulley under predetermined tension that includes elements operating in the pulley's direction of travel.
2. Discussion of Related Art
Tensioning devices of this nature are preferably used in motorcar engines and, as a rule, generate a constant tension in the drive belt under changing operating conditions of the engine. The tension provided by tensioning devices mounted, for example, in a swivable fashion, is generated by means of a spring, which provides automatic compensation when the drive belt is either slack or too taut.
Over the long run, the tension of the drive belt will change between the cold and warm phases of engine operation, due to fatigue of the belt material. During engine operation, it will change due to surge pulses when loads on the generator, vacuum pump, compressor, etc., are turned on and off, and during changes in the load on the engine, or when the vehicle shakes due to poor road conditions. In addition, the idler pulley is affected by the constant vibration of the drive belt caused by the timing gear, combustion-induced effects on the travel of the belt, etc. For these reasons it is recommended that the operating motion of the tensioning device be damped.
However, it is difficult to achieve effective damping because the aforementioned vibrations of the drive belt cause relatively small swivel motion amplitudes in the tensioning device at high frequencies. On the other hand, surge pulses produce overly abrupt, often isolated excursions of the tensioning device.
U.S. Pat. No. 4,283,181, for example, discloses a tensioning device whose swivel bearing is equipped with a friction sleeve having a predetermined radial tension. As a result, a damping friction in both swivel directions of the tensioning device is achieved. A strong radial predetermined tension can provide effective damping of the surge pulses. However, the tensioning device will act like a fixed, immobile guide roller in response to vibration. This means that the vibrations of the drive itself, which may lead to severe wear and tear and premature breakdown, must still somehow be compensated.
German patent No. DE-GM 7 703 393 discloses a hydraulic shock damper which damps the operating motion of the tensioning device hydraulically. In a particular embodiment, a valve is, installed in the fluid channel which, in pulse dampers, achieves excellent damping action in one direction and little damping action in the other. Shock dampers of this type are particularly advantageous for damping large motions with relatively long periods, and are therefore particularly desirable for dissipating the energy in the aforementioned surge pulses. However, they can only be used under certain conditions for damping vibrations. After a short period of operation within a relatively small amplitude range a certain amount of extra clearance space appears in the damper, caused by leakage. Damping is no longer possible in this space. After this has occurred, vibrations of the belt drive are no longer damped in this embodiment. In addition, the usually limited space conditions in tensioning devices allow only short, telescopic shock dampers and, for this reason, the swivel motion must be transferred to the shock damper after reduction. As a result, the aforementioned inoperative extra clearance occupies a considerable part of the motion range of the idler pulley. As a result, the shock damper is at least ineffective in the presence of small swivel motions, and vibrations, in particular.